In plain words
Whatever being is born, moving or unmoving, know it to arise from the conjunction of the field and the knower of the field, best of Bharatas.
Word for word
Whatever being is born, unmoving or moving, know that [to be] from the union of kṣetra and kṣetrajña, O best of the Bharatas.
The commentators
Bhagavadgītābhāṣyam
क्षेत्रज्ञेश्वरैकत्वविषयं ज्ञानं मोक्षसाधनम् ‘यज्ज्ञात्वामृतमश्नुते’(भ. गी. १३ । १२) इत्युक्तम् , तत् कस्मात् हेतोरिति, तद्धेतुप्रदर्शनार्थं श्लोकः आरभ्यते —
यावत् यत् किञ्चित् सञ्जायते समुत्पद्यते सत्त्वं वस्तु ; किम् अविशेषेण ? नेत्याह — स्थावरजङ्गमं स्थावरं जङ्गमं च क्षेत्रक्षेत्रज्ञसंयोगात् तत् जायते इत्येवं विद्धि जानीहि भरतर्षभ ॥
कः पुनः अयं क्षेत्रक्षेत्रज्ञयोः संयोगः अभिप्रेतः ? न तावत् रज्ज्वेव घटस्य अवयवसंश्लेषद्वारकः सम्बन्धविशेषः संयोगः क्षेत्रेण क्षेत्रज्ञस्य सम्भवति, आकाशवत् निरवयवत्वात् । नापि समवायलक्षणः तन्तुपटयोरिव क्षेत्रक्षेत्रज्ञयोः इतरेतरकार्यकारणभावानभ्युपगमात् इति, उच्यते — क्षेत्रक्षेत्रज्ञयोः विषयविषयिणोः भिन्नस्वभावयोः इतरेतरतद्धर्माध्यासलक्षणः संयोगः क्षेत्रक्षेत्रज्ञस्वरूपविवेकाभावनिबन्धनः, रज्जुशुक्तिकादीनां तद्विवेकज्ञानाभावात् अध्यारोपितसर्परजतादिसंयोगवत् । सः अयं अध्यासस्वरूपः क्षेत्रक्षेत्रज्ञसंयोगः मिथ्याज्ञानलक्षणः । यथाशास्त्रं क्षेत्रक्षेत्रज्ञलक्षणभेदपरिज्ञानपूर्वकं प्राक् दर्शितरूपात् क्षेत्रात् मुञ्जादिव इषीकां यथोक्तलक्षणं क्षेत्रज्ञं प्रविभज्य ‘न सत्तन्नासदुच्यते’(भ. गी. १३ । १२) इत्यनेन निरस्तसर्वोपाधिविशेषं ज्ञेयं ब्रह्मस्वरूपेण यः पश्यति, क्षेत्रं च मायानिर्मितहस्तिस्वप्नदृष्टवस्तुगन्धर्वनगरादिवत् ‘असदेव सदिव अवभासते’ इति एवं निश्चितविज्ञानः यः, तस्य यथोक्तसम्यग्दर्शनविरोधात् अपगच्छति मिथ्याज्ञानम् । तस्य जन्महेतोः अपगमात् ‘य एवं वेत्ति पुरुषं प्रकृतिं च गुणैः सह’(भ. गी. १३ । २३) इत्यनेन ‘विद्वान् भूयः न अभिजायते’ इति यत् उक्तम् , तत् उपपन्नमुक्तम् ॥ २६ ॥
In plain words
It has already been said that the knowledge whose one subject is the oneness of the knower of the field and the Lord is the means to liberation — 'knowing which, one reaches the deathless.' But for what reason is this so? To show that reason, the next verse is taken up —
Whatever comes into existence, anything at all — does it just happen, with no rhyme or reason to it? No, says the verse. Whether it's something that never moves, like a tree, or something that does, like an animal — everything that's born comes from one and the same source: the meeting of the field and the one who knows the field. Know this clearly, best of the Bharatas.
So what exactly is this 'conjunction' between the field and its knower supposed to mean? It can't be the ordinary kind of joining — like a rope tied to a pot — because that requires parts to join, and the field-knower, like space itself, has no parts at all. It can't be inherence either, the way threads inhere in cloth, because that kind of bond needs a cause-and-effect relationship between the two things, and nothing like that exists between the field and its knower here. So what is it, then? It's this: the field and its knower, object and subject, utterly different in nature, seem joined only because their properties get mixed up with each other in the mind — exactly the way a rope, in dim light, gets mistaken for a snake, or a shining shell for a piece of silver. That confusion happens only because the real difference between rope and snake, shell and silver, was never clearly seen. The so-called conjunction between field and field-knower is exactly that kind of mistake — nothing but false knowledge wearing a disguise. And here is how scripture leads a person out of that mistake. First comes a clear discrimination between what the field is and what its knower is — carefully separating the two, the way you'd draw the soft reed-pith out from its outer sheath. Then, through the verse that says 'it can't be called existent, and it can't be called non-existent,' the object of knowledge is seen stripped of every label and condition, seen simply as Brahman itself. And the field, meanwhile, is seen for what it really is — no more solid than an elephant conjured by a magician, a scene from a dream, or a shimmering phantom city: it only appears real, it never actually is. Once someone has arrived at this settled understanding, false knowledge simply cannot survive next to it — the two are direct opposites, and the true seeing wins. And since false knowledge was the actual cause of birth, once it's gone, the cause is gone too. That's exactly what makes the earlier statement true: the one who knows puruṣa and prakṛti, together with the guṇas, exactly as described — that knower is not born again. Everything that was claimed has now been shown to follow, soundly and rightly.
Close to the Sanskrit
The knowledge whose object is the oneness of the field-knower and the Lord is the means to liberation — 'knowing which one attains the deathless' (Gītā 13.12) — thus it has been said. From what cause is that so? Thus, for the purpose of showing the cause of that, the verse is begun —
Whatever thing, any being at all, comes into being, arises — is this without any distinguishing factor? He says: no. The unmoving and the moving — the unmoving, and the moving — that is born from the conjunction of the field and the field-knower: thus know, O best of the Bharatas.
What, again, is this conjunction of kṣetra and kṣetrajña that is meant? In the first place, a particular relation of conjunction, mediated by the joining of parts — as with a rope and a pot — is not possible between kṣetra and kṣetrajña, because of the latter's partlessness, like space. Nor is it characterized by inherence, as between thread and cloth, because a mutual cause-effect relation between kṣetra and kṣetrajña is not accepted. To this it is replied: between kṣetra and kṣetrajña, object and subject, of differing natures, the conjunction is characterized by their mutual superimposition of each other's properties, grounded in the absence of discrimination of the true nature of kṣetra and kṣetrajña — like the superimposed conjunction of a snake or silver, due to the absence of that discriminating knowledge, in the case of a rope, mother-of-pearl, and the like. This very conjunction of kṣetra and kṣetrajña, whose nature is superimposition, is characterized by false knowledge. According to scripture, preceded by thorough discrimination of the defining marks of kṣetra and kṣetrajña, having separated out — as one draws a reed-stalk from its sheath — kṣetrajña, already characterized, from kṣetra, whose form was already shown, one who, by the verse 'it is called neither being nor non-being' (Bhagavad Gītā 13.12), sees the object of knowledge, stripped of every particular limiting condition, in its own nature as Brahman — and who has come to this settled understanding regarding kṣetra: 'it merely appears as if existent, though it is truly non-existent', like an elephant conjured by magic, a thing seen in a dream, a phantom city, and the like — for him, because this is opposed to the correct vision just described, false knowledge departs. Because that false knowledge, the cause of birth, has departed, what was said by the verse 'he who thus knows puruṣa and prakṛti together with the guṇas' (Bhagavad Gītā 13.23) — namely, 'the knower is not born again' — that has indeed been correctly and soundly stated.